Try clearing site cookies.

Does it still happen in Troubleshoot Mode?

It didn't work, still, as long I am logged in, can not watch any of the videos.

Did you check the settings in your YouTube account?

You can remove all data stored in Firefox for a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox for this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data for that domain that you do not want to lose, make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website, data for that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

checked all that and nothing happened. YT works on my phone, but on my computer wenn I am logged in.

Go to Youtube Settings > Playback and performance > AV1 settings > Set to "Auto (recommended)".

Are there any errors in the Web Console (Ctrl+Shift+K)?

Does it still happen in a new profile? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.